Author Topic: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind  (Read 5595 times)

c_cld

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c_cld

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c_cld

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djj

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #18 on: May 06, 2014, 05:22:58 pm »
Golly ::)! JT's OotD has been exceedingly fruitful ;D!

NGC3314

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #19 on: May 06, 2014, 08:10:36 pm »
Diffraction spikes. Astronomers have mostly hated them for a couple of centuries, especially in equatorially mounted telescopes where if your object is once wiped out by a spike from a bright star, it will always be. For the numerically minded, the diffraction pattern can be calculated from the Fourier transform of the telescope aperture (the pupil, to be pedantic, since obstructions at the camera end also matter). They are always there, and are part of the point-spread function or PSF, but typically comprise such a small fraction of the object's light that they show up only for very bright objects (I've seen them from planets, and at least one planetary nebula is bright enough to show them on Palomar Sky Survey photographs). This means that a single support (like in small Newtonian telescopes with a single vane supporting the secondary mirror) give two oppositely directed spikes. Four give the familiar cross pattern, and either 3 or 6 evenly spaced obstructions give a hexagonal set (as in images from the Keck telescopes). Planetary observers long favored refractors for seeking low-contrast detail because they don't have diffraction spikes, sometimes pursuing such exotica as off-axis reflecting arrangements (no obstructions) or curved mirror supports. These don't make diffraction spikes, but they do diffract the same amount of light into a round, diffuse halo, which may not be any better.

Sometimes the spikes can at least be put to use. In observing faint companions to bright stars, if possible, observers want the companion to appear between the spikes. One famous picture of Sirius B used a hexagonal mask over the telescope aperture to put the light into 6 spikes deliberately oriented to lave the white dwarf between them. This also means that such observations with an altazimuth telescope, or with Hubble, have scheduling restrictions to do the same thing. Sometimes you'll see a Hubble image composed of data (say in different fillters) taken at different times, in which the spikes in different filters are at different angles.

One of my long-ago professors used to show visitors the companion of Sirius with Vanderbilt University's 0.6m telescope when it was as seen farthest from the bright main-sequence star. He broke up the diffraction pattern by gluing small black tabs to the support vanes, at locations derived from a table of random numbers so this change wouldn't introduce any systematic diffraction structure.

djj

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #20 on: May 06, 2014, 09:38:36 pm »
Fascinating stuff. Many thanks.

c_cld

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #21 on: May 11, 2014, 03:57:27 pm »
QSO on a SDSS diffraction spike!
SDSS J142109.59+010748.2 587726014010622087



badly recorded in DR8-DR10 1237651735235526776


but well selected in GAMA survey
GAMAJ142109.59+010748.2


 8)

JeanTate

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #22 on: May 23, 2014, 07:35:51 pm »
Thanks to NGC3314 and c_cld for terrific additions to this thread!  ;D I particularly like the story of very purposefully gluing things on a spider, at random, but a random that was very carefully designed  8)

Here's something else about diffspikes you may find interesting (I'm quoting in full; source to follow):

Quote
2013-11-16

extract low-resolution spectra from diffraction spikes

In imaging from a telescope with a secondary on a spider (for example, in HST imaging), bright stars show diffraction spikes. More generally, the outer parts of the point-spread function are related to the Fourier Transform of the small-scale features in the entrance aperture. The scale at which this Fourier Transform imprints on the focal plane is linearly related to wavelength (just as the angular size of the diffraction-limited PSF goes as wavelength over aperture).

This means that the diffraction spikes coming from stars contain low-resolution spectra of those stars! That is, you ought to be able to extract spectral information from the spikes. It won't be good, but it should permit measurements of colors or temperatures or SED slopes with even single-band imaging, and aid in star–quasar classification. Indeed, in HST press-release images, you can see that the diffraction spikes are little "rainbows" (see below).

The project is to take wide-band imaging from HST, in fields where stars have been measured either in multiple bands or else spectroscopically, and show that some of the scientific results could have been extracted from the single, wide band directly using the diffraction features.


It's from Hogg's Ideas, by David Hogg. If the name seems somewhat familiar, but you can't quite place him, try searching "Hogg" in this forum (no spoilers). Oddly, "Hogg" gets no hits in GZ Talk ...

Do you think something similar could be done, with diffspikes in SDSS images?

JeanTate

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #23 on: May 23, 2014, 08:09:28 pm »

That's from the first post of this thread, the OotD itself.

If the spiders holding the secondary in an alt-azimuth telescope are at 45° to the horizontal, as they appear to be in the SDSS telescope, and if normal SDSS images always have N to the top (and W to the right), as shown in the last image, under what circumstances could you get vertical/horizontal (NS/WE) diffspikes like in the first image?

For a telescope much like SDSS, located at ~32° N latitude (i.e. not on the equator, or at one of the poles, or even at 45° latitude); and for real stars (i.e. can't point the telescope lower than ~10° altitude).

JeanTate

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #24 on: June 13, 2014, 12:44:55 pm »
QSO on a SDSS diffraction spike!
SDSS J142109.59+010748.2 587726014010622087



badly recorded in DR8-DR10 1237651735235526776

A nice contrast might be an AGN* - not a QSO - with diffspikes (zutopian already posted a QSO with diffspikes), next to a star close to it in the same field (SDSS J144445.95+360709.6). They're faint, but I don't think it's just my imagination  ;)



* It's SDSS J144445.92+360622.0

zutopian

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Re: Sunday, 4 May, 2014: Diffraction Spikes, of the SDSS Kind
« Reply #25 on: June 15, 2014, 07:39:11 am »
Copy from another topic.:

Can a galaxy with an r-band (integrated, model) magnitude of only 17.60 have a nucleus so point-like and so bright that it generates diffspikes (diffraction spikes) in an SDSS image?

Check out SDSS J145854.05+051133.5 (DR7 ObjId 587736545774076154, DR8/9 1237662266998980792); first as a Quench Project object (AGS0000432)*, then DR7, and DR9:







The spectrum shows very strong H-alpha (and [NII]?) emission, but not exceptionally so:



What about nearby (bright) stars, do they have diffspikes with the same orientation? How about nearby galaxies with bright nuclei? Here is the field**, with 'our galaxy' in the center; I'll look at each of the marked objects below (the DR9 image of 'our galaxy' - same scale - is to the right of each line):






Maybe I should do an OOTD on this, and similar galaxies? What do you think?

* not previously posted in the GZ forum before
** I forgot to mark the boundaries of the Fields; however, all the marked objects - and 'our galaxy' - are in the same Field

I found following object with faint diffraction spike, but it is in a different field.:


http://skyserver.sdss3.org/public/en/tools/explore/summary.aspx?id=1237659145633398789

« Last Edit: June 17, 2014, 03:35:15 am by zutopian »